Risk-informed In-service Inspection Research Articles

Reliability Analysis of PAUT Based on the Round-Robin Test for Pipe Welds with Thermal Fatigue Cracks.

Thermal fatigue cracks occurring in pipes in nuclear power plants pose a high degree of risk. Thermal fatigue cracks are generated when the thermal fatigue load caused by local temperature gradients is repeatedly applied. The flaws are mainly found in welds, owing to the effects of stress concentration caused by the material properties and geometric shapes of welds. Thermal fatigue pipes are classified as targets of risk-informed in-service inspection, for which ultrasonic testing, a volumetric non-destructive testing method, is applied. With the advancement of ultrasonic testing techniques, various studies have been conducted recently to apply the phased array ultrasonic testing (PAUT) method to the inspection of thermal fatigue cracks occurring on pipes. A quantitative reliability analysis of the PAUT method must be performed to apply the PAUT method to on-site thermal fatigue crack inspection. In this study, to evaluate the quantitative reliability of the PAUT method for thermal fatigue cracks, we fabricated crack specimens with the thermal fatigue mechanism applied to the pipe welds. We performed a round-robin test to collect PAUT data and determine the validity of the detection performance (probability of detection; POD) and the error in the sizing accuracy (root-mean-square error; RMSE) evaluation. The analysis results of the POD and sizing performance of the length and depth of thermal fatigue cracks were comparatively evaluated with the acceptance criteria of the American Society of Mechanical Engineers Code to confirm the effectiveness of applying the PAUT method.

Probabilistic Safety Analysis and Risk-Based Inspection of nuclear research reactors: state-of-the-art and implementation proposal

Industrial facilities systems deteriorate over time, thus increasing the possibility of accidents. Risk-Based Inspection (RBI) classifies such systems by their risk information with the purpose of prioritizing inspection efforts. RBI can reduce inspection activities, resulting in lower risk levels, and maintaining reliability and safety in acceptable levels. Risk-Informed In-Service Inspection (RI-ISI) is a RBI approach used in nuclear industry. RI-ISI uses outcomes from Probabilistic Safety Analysis (PSA) of Nuclear Power Plants (NPP) to plan In-Service Inspections (ISI). Despite nuclear research reactors are simpler and have lower risks than power reactors, the application of PSA to them may be useful for safety improvements once they are more flexible, provide easier access to its core and allow changes in fuel configurations. Ageing management of components is also required to assure continued adequacy of safety levels, reliable operation, and compliance with operational limits and conditions. This includes periodic review of ISI programs in which monitoring of material deterioration is considered, and that can be supported by the RBI approach. A review of state-of-the-art of PSA and RBI and an implementation proposal applied to nuclear research reactors is presented in this work, as well as its application to a TRIGA reactor using computer codes developed by ReliaSoft® Corporation.

Development of risk-based inspection programme for BARC Research Reactors–Dhruva: experience and insights

As part of defence-in-depth philosophy an In-Service Inspection programme for research reactors at BARC has been implemented. The current ISI methodology is based on deterministic approach through the application of rules and criteria provided in ASME Section 11 and Section 5. For the research reactor Dhruva two campaigns of ISI have been completed. The risk-informed approach is widely employed for Nuclear Power Plants; however, there is no published literature on Risk-informed ISI for a research reactor. Development work on risk-informed ISI has been performed for Dhruva and a document has been prepared and submitted to safety committees for review and approval. The objective is to utilize the available Level 1 PSA and ISI insights to optimize the ISI programme. This paper, discusses the development approach and insights gained from this project and provides recommendation on initiating activities to encourage other facilities to consider implementation of RI-ISI for research reactors.

A Probabilistic Environmentally Assisted Cracking Model for Steam Generator Tubes

A probabilistic environmentally assisted cracking (PEAC) model was developed to describe the propagation of primary water stress corrosion cracking for Alloy 600 in roll-transition region of steam generator (SG), a severe environmentally assisted cracking problem in pressurized water reactors (PWRs). In the PEAC model, crack growth rate (CGR) and probability of failure (POF) were obtained by adopting a Bayesian inference that decreases the uncertainties of unknown parameters and their distributions in theoretical equations. The CGR is mainly dependent on three factors: probability of detection (POD), initial crack size distribution, and stress distribution. The POD, which is a logistic link was updated with Bayesian inference based on SG inspection data. The crack size distribution, which is relative to initiation time expressed by a Weibull function, was also updated with Bayesian inference using POD. The stress distribution caused by mechanical rolling is considered to be a major contributing factor along the SG tube. It based on finite element analysis is deterministic model unlike POD and initial crack distribution. According to this model, the uncertainty of hyperparameters in the CGR which are parameters of a prior distribution was reduced, and the appropriate level of confidence was achieved by utilizing the available data. Moreover, a benchmark study for the SG tube was performed to evaluate reliability of Alloy 600 SG components in nuclear power plants. The POF was estimated from the developed PEAC model and failure criteria by taking into account the effects of inspection and repair of defective tubes. The results from this study are applied to demonstrate risk reduction in PWRs by adopting risk-informed in-service inspection.

Guidelines for Integrity and Lifetime Assessment of Components and Piping in WWER NPPs during Operation (VERLIFE)

VERLIFE – “Unified Procedure for Lifetime Assessment of Components and Piping in WWER NPPs during Operation “was developed within the 5th Framework Programme of the European Union in 2003 and later upgraded within the 6th Framework Programme “COVERS – Safety of WWER NPPs” of the European Union in 2008.This Procedure has to fill the gap in original Soviet/Russian Codes and Rules for WWER type NPPs, as these codes were developed only for design and manufacture and were not changed since their second edition in 1989.VERLIFE Procedure is based on these Russian codes but incorporates also new developments in research, mainly in fracture mechanics, and also some principal approaches used in PWR codes.To assure that VERLIFE Procedure will remain a living document, new 3-years IAEA project (in close co-operation with another project 6th Framework Programme of the European Union “NULIFE – Plant Life Management of NPPs”) started in 2009.Within this project, upgrading/updating of the VERLIFE procedure is prepared together with the extension by (at least) following procedures:-Lifetime of reactor pressure vessel internals-Leak-before-break concept for WWER NPPs-No-break-zone for WWER NPPs-Probabilistic approach for failure and lifetime assessment of WWER reactor pressure vessels and primary piping (including Risk-informed ISI)-Qualification of NDT procedures-Thermal fatigue evaluation-Supports of components and pipingFinal document, after its approval by expert groups of the IAEA and NULIFE, will be issued in 2014 as “IAEA VERLIFE Guidelines for Integrity and Lifetime Assessment of Components and Piping in WWER NPPs”.

Optimal sample size for probability of detection curves

The use of probability of detection curves to quantify the reliability of non-destructive examination (NDE) systems is common in the aeronautical industry, but relatively less so in the nuclear industry, at least in European countries. Due to the nature of the components being inspected, sample sizes tend to be much lower. This makes the manufacturing of test pieces with representative flaws, in sufficient numbers, so to draw statistical conclusions on the reliability of the NDT system under investigation, quite costly. The European Network for Inspection and Qualification (ENIQ) has developed an inspection qualification methodology, referred to as the ENIQ Methodology. It has become widely used in many European countries and provides assurance on the reliability of NDE systems, but only qualitatively. The need to quantify the output of inspection qualification has become more important as structural reliability modelling and quantitative risk-informed in-service inspection methodologies become more widely used. A measure of the NDE reliability is necessary to quantify risk reduction after inspection and probability of detection (POD) curves provide such a metric. The Joint Research Centre, Petten, The Netherlands supported ENIQ by investigating the question of the sample size required to determine a reliable POD curve. As mentioned earlier manufacturing of test pieces with defects that are typically found in nuclear power plants (NPPs) is usually quite expensive. Thus there is a tendency to reduce sample sizes, which in turn increases the uncertainty associated with the resulting POD curve. The main question in conjunction with POS curves is the appropriate sample size. Not much guidance on appropriate sample sizes can be found in the published literature. Often only qualitative advice is given.The aims of the work summarized in this paper were to develop numerical simulations to determine appropriate and effective inspection target sizes, their number, and distribution to produce valid POD vs. size curves, and to summarize these findings as guidelines for the NDE practitioner in designing an experiment to assess system inspection effectiveness.

Application of Risk-Informed Inspection Strategy to Improve the Lifetime and Efficiency of Cleaning Pigs

The aim of the research work was to analyze the operation of cleaning pigs, in natural gas transportation pipelines from point of view of risk-informed in-service inspection. During the analysis, the main factors considered were the stresses (mechanical, thermal and environmental) and loads, which have effect on cleaning pigs during its operation. The possible damage mechanisms were analyzed which can occur due to their consequences and qualitative probability analysis was performed on the basis of the available information.

A study on the effect of flaw detection probability assumptions on risk reduction achieved by non-destructive inspection

Leakages and ruptures of piping components lead to reduction or loss of the pressure retaining capability of the system, and thus contribute to the overall risk associated with nuclear power plants. In-service inspection (ISI) aims at verifying that defects are not present in components of the pressure boundary or, if defects are present, ensuring that these are detected before they affect the safe operation of the plant. Reliability estimates of piping are needed e.g., in probabilistic safety assessment (PSA) studies, risk-informed ISI (RI-ISI) applications, and other structural reliability assessments. Probabilistic fracture mechanics models can account for ISI reliability, but a quantitative estimate for the latter is needed. This is normally expressed in terms of probability of detection (POD) curves, which correlate the probability of detecting a flaw with flaw size. A detailed POD curve is often difficult (or practically impossible) to obtain. If sufficient risk reduction can be shown by using simplified (but reasonably conservative) POD estimates, more complex PODs are not needed. This paper summarises the results of a study on the effect of piping inspection reliability assumptions on failure probability using structural reliability models. The main interest was to investigate whether it is justifiable to use a simplified POD curve. Further, the study compared various structural reliability calculation approaches for a set of analysis cases. The results indicate that the use of a simplified POD could be justifiable in RI-ISI applications.

OS0818 原子力発電施設の損傷事例データベースを用いた損傷モードの分析 : 溶接部の疲労損傷事例の調査

In this study, event description, root cause analysis and corrective action of fatigue failure of piping weld were reviewed using domestic nuclear information archives (NUCIA) and the Press Releases of each electric power company. For the application of Risk Informed In-Service Inspection (RI-ISI), fatigue mode of field experience in the weldment of nuclear power plant components was analyzed.

Application of piping failure database to nuclear safety issues in Korea

This paper describes analyses and application of OPDE (OECD Piping Data Failure Exchange) database and NPFD (Nuclear Piping Failure Database) to some safety related issues for Korea nuclear power plants. Safety issues considered in this study include the piping rupture frequency assessment, the screening criteria of the leak before break application, and the augmented in-service inspection and the risk-informed ISI (RI-ISI). As a result of this study: (1) Piping rupture frequencies of the very small LOCA, feedwater line break events, and flood events were evaluated and used as initiating event frequencies for the PSA of Korean PWR plants; (2) Based on the review of OPDE DB and LBB screening criteria, LBB application to the pressurizer surge line, safety injection system, and shutdown cooling system were approved under the conditions that detailed thermal stress analyses should be performed to the systems. (3) For RI-ISI application, major degradation mechanisms in the reactor coolant system were analyzed based on the analysis of the OPDE and NPFD and augmented inspections were enforced to the pipe segments susceptible to degradation mechanism such as thermal stratification/fatigue.

Derivation and use of probability of detection curves in the nuclear industry

The use of probability of detection curves to quantify NDT reliability is common in the aeronautical industry, but relatively less so in the nuclear industry, at least in European countries. The main reason for this lies in the very nature of the components being inspected. Sample sizes of inspected flaws tend to be much lower and it is often very difficult to procure or manufacture representative flaws in test-pieces in a high enough number to draw statistical conclusions on the reliability of the NDT system being investigated. Similar considerations led to the development of the ENIQ inspection qualification methodology, based on the idea of the technical justification, ie a document assembling evidence and reasoning providing assurance that the NDT system is indeed capable of finding the flaws which it is designed to detect. The ENIQ methodology has become widely used in many European countries and is gaining appreciation outside Europe as well, but the assurance it provides is usually of a qualitative nature. The need to quantify the output of inspection qualification has become more and more important, especially as structural reliability modelling and quantitative risk-informed in-service inspection methodologies become more widely used. To credit the inspections in structural reliability evaluations, a measure of the NDT reliability is necessary. A probability of detection (POD) curve provides such metric. The purpose of this paper is to briefly review the statistical models proposed to quantify NDT reliability, to highlight the potential problems that can arise if the main underlying assumptions and requirements are not verified and to clarify the confusion that can arise over the nature of the POD curve and associated confidence bounds.

Bayesian Analysis of Piping Leak Frequency Using OECD/NEA Data

The estimation of piping failure frequency is an important task to support the probabilistic risk analysis and risk-informed in-service inspection of nuclear power plant systems. This paper describes a hierarchical or two-stage Poisson-gamma Bayesian procedure and applies this to estimate the failure frequency using the Organization for Economic Co-operation and Development/Nuclear Energy Agency pipe leakage data for the United States nuclear plants. In the first stage, a generic distribution of failure rate is developed based on the failure observations from a group of similar plants. This distribution represents the interplant (plant-to-plant) variability arising from differences in construction, operation, and maintenance conditions. In the second stage, the generic prior obtained from the first stage is updated by using the data specific to a particular plant, and thus a posterior distribution of plan specific failure rate is derived. The two-stage Bayesian procedure is able to incorporate different levels of variability in a more consistent manner.

Activities on risk-informed in-service inspection within the ENIQ network

The European Network for Inspection and Qualification (ENIQ) is a utility-driven network that works towards a harmonised European approach on reliable and effective in-service inspection (ISI), primarily in the nuclear power industry. The ENIQ has two task groups: a Task Group on Qualification (TGQ), which works on issues related to the qualification of non-destructive evaluation (NDE) systems, and a Task Group on Risk (TGR), which is focused on risk-informed in-service inspections (RI-ISI). After publishing the European Framework Document on RI-ISI in 2005, ENIQ TGR has been working on producing more detailed recommended practices and discussion documents on several RI-ISI related issues. The following reports have been issued: (1) Discussion document on the role of ISI within the philosophy of defence in depth; (2) Recommended practice on verification and validation of structural reliability models (SRMs) and associated software to be used in RI-ISI programmes; (3) Recommended practice on guidance on expert panels in RI-ISI; (4) Discussion document on application of RI-ISI on reactor pressure vessel. Work is ongoing to develop a discussion document on the updating of RI-ISI programmes and new initiatives were recently launched to study topics such as what magnitude of risk reduction is reasonable to achieve through ISI, how to set inspection targets following the selection of ISI sites and how to produce detection probability estimates for ISI. In addition, TGR has been active in initiating international projects linked closely to its work, such as the JRC-OECD/ NEA co-ordinated RI-ISI benchmark exercise (RISMET) and the project on the relation between inspection qualification and RI-ISI. The aim of this paper is to review and explain these activities of ENIQ TGR.

Study on Risk-Informed In-Service Inspection for BWR Piping

Risk-informed in-service inspection for piping was studied for a BWR plant. Piping segment rupture probabilities were determined by Bayesian transform from piping failure events in the database of the OECD-NEA Piping Failure Data Exchange project. Based on the methodology of the Westinghouse Owners Group, core damage frequency induced by each segment rupture was determined by the use of a surrogate component in the PSA model. Nondestructive examinations were added to leak examinations for segments of the resultant high risk significance. The changes from current examinations gave around 29% reduction of segments subject to both leak and nondestructive examinations within the total segments. Deterministic insights and engineering judgments on top of risk significance should be applied to obtain the final decision of inspection methods. An extent-of-examination was studied by the adoption of the Perdue-Abramson model in the Westinghouse Owners Group methodology. The necessary leak frequency of a crack in a segment was calculated by the probabilistic fracture mechanics code PRAISE. Two segments of high risk significance showed lower or slightly higher extents-of-examination, respectively, than the current extent-of-examination. To contribute to the enhancement of the scientific rationality of piping inspections, technical knowledge was accumulated.

A Stochastic Model for Piping Failure Frequency Analysis Using OPDE Data

The accurate estimation of piping failure frequency is an important task to support the probabilistic risk assessment and risk-informed in-service inspection of nuclear power plants. Although probabilistic models have been reported in the literature to analyze the piping failure frequency, this paper proposes a stochastic point process model that incorporates both a time dependent trend and plant-specific (or cohort) effects on the failure rate. A likelihood based statistical method is proposed for estimating the model parameters. A case study is presented to analyze the Class 1 pipe failure data given in the OPDE Database.

Overview on the different applications of probabilistic safety assessment for nuclear power plants

Abstract Worldwide it can be recognised that the use of probabilistic safety assessment (PSA) in regulatory as well as operational decision-making is state of the art and seen as a successful development. Therefore, in most cases the regulator encourages the performance of PSAs to provide information to complement and support the defence in depth philosophy as well as operational configuration decisions. The main application of the PSA is still as part of integrated safety reviews, in particular in the frame of comprehensive (periodic) safety reviews. Other more specific applications areas of PSA are, among others, design evaluation, event analysis with aid of PSA, evaluation of technical specifications; risk-informed in-service inspection, risk monitoring and accident management. The extent of these applications vary from country to country but has been increasing during the last years.

The link between risk-informed in-service inspection and inspection qualification

A growing need exists for a quantitative measure of inspection effectiveness as an input to quantitative riskinformed in-service inspection (RI-ISI). A Probability of Detection (POD) curve could provide a suitable metric, but there can be significant problems associated with generating realistic POD curves by practical trials. The ENIQ inspection qualification methodology is used to provide high confidence that an inspection system will achieve its objectives, but is not designed to provide a quantitative measure of the type that can be used in RI-ISI analysis. This paper describes the results of a project set up to investigate approaches to quantifying the confidence associated with inspection qualification. The project applied quantification approaches in a pilot study and produced guidelines on how to relate inspection qualification results, risk reduction and inspection interval.

Study on Risk-Informed In-Service Inspection for BWR Piping

Risk-informed in-service inspection for piping was studied for a BWR plant. Piping segment rupture probabilities were determined by Bayesian transform from piping failure events in the database of the OECD-NEA Piping Failure Data Exchange project. Based on the methodology of the Westinghouse Owners Group, core damage frequency induced by each segment rupture was determined by the use of a surrogate component in the PSA model. Nondestructive examinations were added to leak examinations for segments of the resultant high risk significance. The changes from current examinations gave around 29% reduction of segments subject to both leak and nondestructive examinations within the total segments. Deterministic insights and engineering judgments on top of risk significance should be applied to obtain the final decision of inspection methods. An extent-of-examination was studied by the adoption of the Perdue-Abramson model in the Westinghouse Owners Group methodology. The necessary leak frequency of a crack in a segment was calculated by the probabilistic fracture mechanics code PRAISE. Two segments of high risk significance showed lower or slightly higher extents-of-examination, respectively, than the current extent-of-examination. To contribute to the enhancement of the scientific rationality of piping inspections, technical knowledge was accumulated.

Study on Practical Application of Risk Informed In-Service Inspection (Case Study on Class 1 Piping of a Boiling Water Reactor)

This paper describes pilot study relevant to the application of Risk Informed In-Service Inspection (RI-ISI) in a BWR plant. This study conducted for typical BWR5 plant of Tokyo Electric Power Company, and partial application of RI-ISI limited in class 1 piping was carried out. A series of studies related to RI-ISI-segments division process, assessment of degradation possibility in segments, failure probability assessment of segments, risk assessment, risk significant assessment and ISI planning-was performed. The inspected portion and its frequency at the time of applying RI-ISI was decided so as to not exceed core damage probability (CDF) at the time of applying the conventional inspection method. As a result, volumetric examination in ISI will not required in 70% of segments (47 segments in 68 segments), and it suggests that the application of RI-ISI is much effective for reduction of radioactive exposure of inspectors without increase of risk by introducing RI-ISI.

Updating of risk-informed ISI programme for Ignalina RBMK-1500 nuclear power plant in Lithuania: Results and challenges

The paper presents a risk-informed in-service inspection (RI-ISI) pilot study project of 300 mm piping at Ignalina nuclear power plant (INPP) RBMK-1500 reactor, located in Lithuania. The RI-ISI study investigates optimal 300 mm piping ISI strategies with respect to risk and required resources. In total 1240 stainless steel welds were analyzed, assuming inter-granual stress corrosion cracking (IGSCC) to be the main damage mechanism. Pipe break frequency was estimated by probabilistic fracture mechanics methods and combined with safety barriers, provided by probabilistic safety assessment (PSA) study. After 3 years of operation, updating of RI-ISI was performed by taking into account new statistical data on pipe defects. Comparison with previous RI-ISI program was performed. The paper includes discussion on uncertainties in the study and robustness of RI-ISI programs.